Process for the production of separable deposits of iodine chromium



Oct. 27, 1964 Filed June '17, 1959 Main Body of deposited Chromium Initially deposited Chromium Layer 3 GEL ION 0F SEPARABLE N- D. PROCESS FOR THE PRODUCT DEPOSITS OF IODINE CHROMIUM nedZone INVENTOR.

I011 D. Voigel United States Patent 3,154,409 PROCESS FOR THE PRODUQTHON OF SEPARABLE DEPOSITS 0F IUDINE CHRQMIUM Neil D. Veigel, Columbus, Ohio, assignor, by mesne assignments, to Chilean Nitrate Sales Corporation, New

York, N.Y., a corporation of New York Filed June 17, 1959, Ser. No. 820,964 Ciaims. (Cl. 7584.4)

This invention relates to the recovery of chromium metal. More particularly, the invention contemplates the provision of an improved process for recovering substantially pure elemental chromium by thermal dissociation of chromium iodide vapors and deposition of chromium on a heated dissociation-deposition surface in the form of a deposit which is readily recoverable from said surface.

It is now well established that certain metals including, for example, titanium, chromium, and zirconium, can be recovered in pure form by contacting their corresponding iodides in the vapor phase with a heated dissociation-deposition filament or surface maintained at a temperature above the dissociation temperature of the metal iodide; such procedure resulting in dissociation of the metal iodide at the dissociation-deposition surface and deposition of the pure metal on that surface. Assuming that an atmosphere of the metal iodide vapor is maintained in contact with the dissociation-deposition surface, the deposition of pure metal can be effected on a continuous basis and a substantial body of metal will be built up progressively on the original surface.

The foregoing classical Van Arkel-De Boer technique as applied to the production and recovery of titanium and zirconium, for example, permits one to effect direct recovery of the deposited pure metal, fused in intimate relationship to the original dissociation-deposition filament or core, which is formed as a directly-heated fine wire or rod of the depositing metal, or as a fine wire of a dissimilar metal such as tungsten or molybdenum. Upon application of this hot-wire technique in eiforts to dissociate chromium iodides for the production and deposition of high-purity chromium on wire dissociation filaments formed of various materials, including chromium, it has been found that the reaction proceeds extremely slowly, because of the limited deposition surface available, and the chromium deposits thus produced tend to consist of coarsely-crystalline, brittle, spiny structures. Furthermore, because of the highly nonuniform nature of these deposits and the consequent large variation in their electrical resistance from point to point, it is difficult to maintain uniform and constant filament temperatures, with the result that filament failures are difficult to avoid.

It has been found heretofore, however, that coherent, compact deposits of high-purity metallic chromium can be produced in commercial yields by dissociatiaon of chromium iodides and deposition of the elemental chromium 0n indirectly-heated dissociation surfaces formed, for eX- ample, of refractory amorphous substances such as quartz, high-silica glass, silica, and the like (see copending US. application Serial Nos. 579,955, now Patent 3,116,144, and 579,970, now abandoned, both filed April 23, 1956). More recently, as described in U.S. Patent No. 2,955,566, issued October 11, 1960, of Ivor E. Campbell and John M. Blocher, Jr., it has been found that high-purity deposits of chromium can be formed on indirectly-heated metallic dissociation-deposition surfaces. Of course, in employing deposition surfaces of the general class described, upon completion of the deposited metal body it is necessary to separate the desired pure metal from the surface upon which it was deposited. In order to expedite this operation, it has been proposed heretofore that the dissociation-deposition surface be formed as a sheath ice which is removably associated with a bayonet or fingerlike resistance heating element (copending applications, supra). The actual recovery can be accomplished by mechanically stripping the deposited metal, as by machining, from the deposition support. Whether or not the dissociation-deposition surface takes the form of a removable sheath or otherwise, it is quite apparent that the recovery procedures necessary in this general type of deposition cycle complicate the overall process, as compared, for example, with the direct recoveries realized by use of directly-heated filament-type dissociation-deposition elements.

It is the principal object of the present invention to provide an improved process for the production of a unique deposited body of metallic chromium which is characterized by the presence therein of a zone or region of weakness that permits the major portion of the metal to be broken away and recovered from the remainder thereof.

The process of the invention is based upon my discovery that distinct stratification of a depositing body of chromium can be effected by means of an abrupt change in the normal deposition conditions during the course of the deposition cycle, whereby the crystal growth habit of the depositing chromium is altered, establishing a zone or region of weakness within the deposited body. Thus, I utilize the foregoing phenomenon by forming an initial thin layer of deposited chromium on the dissociationdeposition surface, and thereafter effect an abrupt change in the deposition conditions to form a zone or region of Weakness between the initial thin layer and subsequent deposits of chromium which form the main deposited body for recovery upon completion of the overall cycle of operations. The main deposit of chromium may be readily broken away from the initial layer and recovered from the dissociation-deposition surface by application of mechanical forces suiiicient to effect separation of the respective layers along the relatively weakened zone.

In carrying out the process of the invention, 1 may employ apparatus of the general type described above. A more thorough description of suitable dissociation-deposition units may be had by reference to any of the aboveidentified copending applications Serial Nos. 579,955; 579,- 970; or 653,246. In simplified form, the apparatus comprises a suitable reaction vessel in which there is disposed a tubular dissociation-deposition element or sheath closed at one end and adapted to be heated by an electrical resistance unit disposed within the tubular element, the overall assembly being spaced from the reaction vessel wall. The reaction vessel is adapted to receive a charge of crude metallic chromium which is maintained, by means of a suitable perforated liner, in spaced relationship from the surface of the dissociation-deposition element. In operation, a small amount of elemental iodine or chromous iodide is supplied to the reaction vessel and the unit is then sealed, preferably through a suitable outgassing arrangement of the type described in the aforementioned copending applications. Preferably, the system is evacuated to remove air prior to initial heating by a suitable vacuum line, and continued evacuation of the reaction vessel during the initial heating period serves to remove occluded or adsorbed gases which may be evolved. O f course, suitable precautionary measures should be exercised to prevent loss of iodine during the evacuation operation such as by (1) keeping it in a suitably refrigerated side vessel, (2) storing it in a frangible capsule, or (3) adding the necessary iodine in the form of anhydrous chromium iodide which has a much lower vapor pressure. The reaction vessel is then heated to a temperature within the range 550 900 C. to cause iodine and chromium to react with the formation of chromous iodide which, at the temperatures involved, has an appreciable vapor pressure. The tubular dissociation-deposition element is then heated to a dissociation temperature for chromous iodide, preferably within the range 750-1000 C. The chromous iodide vapors will be decomposed at the surface of the dissociation-deposition element depositing metallic chromium thereon and liberating elemental iodine for reaction with the impure metallic charge at the lower temperatures maintained within the reaction vessel, thereby forming additional chromous iodide. There is thus effected a continuous transfer of the chromium from the crude charge to the deposited, pure chnomium body on the dissociation-deposition element, such transfer continuing until the charge is exhausted. In theory, at least, the iodine functions as a carrier to extract chromium from its impurities.

I have found that the crystal habit of the depositing body of chromium can be altered preferably by changing the temperature of the reaction vessel and/ or the temperature of the dissociation-deposition element, and it is this phenomenon which is employed in accordance with the process of the invention to introduce the separatory or weakened zone into the deposited body of chromium.

In actual operation for the production of a deposited body of chromium having a weakened separatory region formed therein, I may operate the dissociation-deposition unit under one set of conditions to form an initial thin layer of deposited chromium on the surface of the dissociation-deposition element, and, thereafter, abruptly change the operation conditions by adjusting the temperatur of the reaction vessel and/ or dissociation-deposition element to a different set of conditions at which the remainder or main portion of the deposited body of chromium is formed. The changed nature of the crystal growth habit under the respective operating conditions will give rise to a weakened zone between the initial thin deposit of chromium and the main body of deposited metal which can be fractured by mechanical force to separate the main deposit from the dissociation-deposition element.

Alternatively, I may operate under on set of conditions to form the initial deposit in contact with the dissociationdeposition element, thereafter abruptly changing the operating conditions and continuing under the changed conditions for a limited period of time to form a very thin intermediate layer of chromium of altered crystal growth habit, as compared with the initial deposit, and thereafter effect another change in the operating conditions, either returning to the condition-s under which the initial deposit was formed or a distinct set of operating conditions, to complete the main deposit of chromium. The overall deposited body of chromium formed in this type of operation will comprise the initial layer in direct contact with the dissociation-deposition surface, the thin intermediate layer constituting the desired weakened zone, and the outermost principal body of metallic chromium which can be separated and recovered by the application of mechanical force suitable to fracture the weakened zone.

In practice, I have found that the desired change in growth habit can be eifected by maintaining the temperature of the dissociation-deposition element constant while increasing or decreasing the normally lower temperature of the reaction vessel from the value at which the initial thin deposit is formed, or I may maintain the reaction vessel temperature substantially constant and abruptly increase or decrease the temperature of the dissociationdeposition element. Alternatively, I may increase or decrease the temperature of the reaction vessel from the temperature employed in the formation of the initial thin layer, while simultaneously increasing or decreasing the operating temperature of the dissociation-deposition element such that the temperature differential existing between the reaction vessel and dissociation-decomposition element is abruptly increased or decreased, thereby giving rise to an altered growth habit for the depositing chromium meta-l. I have found that a particularly distinct weakened zone may be formed "by increasing the temperature of the reaction vessel from an initial low value while decreasing 4 the temperature of the dissociati0n-deposition element from an initial high value to create a reduction of the order of 300 C. in the temperature differential existing between the respective temperatures employed during the formation of the initial deposit. Of course, since 1t is essential that the main portion of the deposition cycle be conducted under conditions of optimum deposition elficiency, one may create the initial thin deposit, or any intermediate weakened deposit, under deliberate condo tions of relatively low efliciency such that th abrupt change in operating conditions necessary for the formation of a weakened region can be effected by simply adjust-ing the operating temperatures of the unit to their optimum respectiv final values for completion of the deposition cycle. It is found that the desired change in crystal growth habit can be realized by relatively slight variations in the deposition conditions, and the adjustment required to accomplish this result is relatively non-critical. On the other hand, one may emphasize therelative degree of weakness of the separatory zone by efiec'ting greater variations in the respective deposition conditions employed in the formation of the initial and final deposits of chmmium, or the intermediate and final deposits of chromium.

The separatory crystal zone introduced between the two layers of iodide chromium will be weakest when the rate of change of operating conditions is infinite. From a practical standpoint, however, it is impossible to change the operating conditions instantaneously, and the actual rate of change obtainable depends on the ther mal capacity of the materials to be heated and the heat= ing elements. Accordingly, the heating elements associated with the reaction vessel and deposition element should be designed so that the requisite change in open ating conditions can be effected as abruptly as possible. For example, in my investigations with a deposition tem-* perature of about 1000 C., I have found it to be en tirely possible to produce a pronounced separatory zone by operating with a bulb temperature of 630 C. for the first four to twelve (4-12) hours of a run and, in about thirty-five minutes, raising the bulb temperature to about 730 C. for the remainder of the run. Of course, it will be understood that a total deposition cycle may be measured in terms of many hours or even days, so that a change in operating conditions eifected over a period of one-half hour, as above, or even several hours, is truly an abrup change in relation to the overall cycle. Thus, as used herein I would define an abrupt tern perature change as a temperature change as rapid as the heat storage capacity of the apparatus and its ability to accommodate itself to temperature changes will permit without damage.

In actual practice, I have found that the deposited crystal habit is most strongly influenced by bulb or reaction vessel temperature changes, and, hence, I prefer to introduce the separatory plane by operating at a constant deposition temperature and simply altering the bulb temperature in the manner illustrated 'hereinbefore.

It is believed that the invention may be best understood by reference to the following specific example taken in-conjunction with the accompanying drawing, wherein the single figure illustrates schematically, in cross-section on an enlarged scale, a typical deposit of chromium formed in accordance with the process of the invention.

Example A conventional reaction vessel (not shown) was charged with crude chromium metal and a small quantity of chromous iodide, evacuated and placed in an oven heated to maintain an initial vessel temperature of 600 C. Concurrently, the heating means 2 was operated to establish a temperature of 1000 C. at the'outer surface of the dissociation-deposition element 1. Continuance of such conditions caused the chromous iodide to vaporize, the iodide then being dissociated at the outer surface ing log:

Reaction Temperature Vessel of Finger 1, Time Interval Tempeature, 0.

Start to 4 hours. 55 mins 600 l, 000 4 hrs. 55 mins. to 5 hrs. 50 mins 730 1,000 5 hrs. 55 mins. to 140 hrs 730 1, 000

Thus, the dissociation-deposition surface was maintained constant at an optimum deposition temperature of 1000 0., whereas the reaction vessel temperature was altered from its initial value of 600 C. after about 5 hours operating time to 730 C. This change was effected over a total elapsed time of 55 minutes, and the unit was then operated under the altered conditions for a total elapsed run of 140 hours.

The finished deposited body of substantially pure elemental chromium consisted of an inner, initial deposit approximately 3 mil in thickness, and an outer main deposit approximately 80 mil in thickness, the two layers being integrally joined by a thin intermediate deposit, representing the weakened zone, wherein the chromium crystals were of decidedly different character than in the remainder of the deposit.

The 80-mil outer layer was easily broken away and separated from the 3-mil inner layer.

While the invention has been described with specific reference to a static-type or closed reaction unit, it will be appreciated that it is equally applicable to a dynamictype deposition unit in which the chromium iodide vapors are formed separately and supplied to the dissociationdeposition site in the form of a positively maintained flowing body of gas. In point of fact, it is postulated that a simple change in the operating pressure of such a system could be used to introduce the desired structural change or separatory plane in the deposited body of metallic chromium.

An attendant advantage of the process of the invention resides in the fact that it permits reuse of the deposition sheaths after a main deposit of metal has been broken away and separated from the initial thin deposit of metal carried on the sheaths.

This application forms a continuation-in-part of my prior co-pending application Serial No. 653,243, filed April 16, 1957.

Having thus described the subject matter of my invention, what it is desired to secure by Letters Patent is:

1. In a process for the production of pure chromium by dissociation and deposition from chromous iodide in contact with a heated dissociation-deposition element within a heated reaction vessel, the improvement that comprises depositing an initial thin layer of chromium on said dissociation-deposition element under a preliminary set of operating conditions with respect to the temperature of said dissociation-deposition element and the temperature of said reaction vessel, and thereafter abruptly changing the preliminary operating temperatures to effect a change in the temperature differential between said dissociation-deposition element and said reaction vessel of the order of from 100 to 300 C. to alter the crystal growth habit of the depositing chromium, whereby a separatory weakened zone is formed between the initial thin layer of chromium and chromium subsequently deposited on said dissociation-deposition element, and separating said subsequently deposited chromium at said separatory weakened zone.

2. In a process for the production of pure chromium by dissociation and deposition from chromous iodide in contact with a heated dissociation-deposition element within a heated reaction vessel, the improvement that comprises depositing an initial thin layer of chromium on said dissociation-deposition element under a preliminary set of operating conditions with respect to the temperature of said dissociation-deposition element and the temperature of said reaction vessel and thereafter abruptly changing the preliminary operating temperatures of said dissociation-deposition element and said reaction vessel to effect a change in the temperature differential between said dissociation-deposition element and said reaction vessel of the order of from to 300 C. to alter the crystal growth habit of the depositing chromium throughout the remainder of the deposition cycle, whereby a separatory weakened zone is formed between the initial thin layer of chromium and subsequently deposited metal, and separating said subsequently deposited metal at said separatory weakened zone.

3. In a process for the production of pure chromium by dissociation and deposition from chromous iodide in contact with a heated dissociation-deposition element Within a heated reaction vessel, the improvement that comprises depositing an initial thin layer of chromium on said dissociation-deposition element under a preliminary set of operating conditions with respect to the temperature of said dissociation-deposition element and the temperature of said reaction vessel, and thereafter abruptly changing the preliminary operating temperatures of said dissociation-deposition element and said reaction vessel to eifect a change in the temperature differential between said dissociation-deposition element and said reaction vessel of the order of from 100 to 300 C., and maintaining the new conditions for a controlled period of time sufiicient to alter the crystal growth habit of the depositing chromium, whereby a thin, weakened intermediate separatory-layer of chromium is formed between the initial thin layer of chromium and chromium subsequently deposited on the dissociation-deposition element, and separating said subsequently deposited chromium at said separatory layer.

4. In the method for obtaining substantially pure chromium by contacting an atmosphere of heated chromium iodide vapor with a dissociation-deposition surface maintained within a heated reaction vessel at a dissociation temperature for chromous iodide and thereby causing a body of substantially pure elemental chromium to be progressively deposited on such surface, the improvement that comprises allowing an initial thin layer of chromium to be progressively deposited upon the dissociation-deposition surface under a preliminary set of operating conditions with respect to the temperature of said dissociation-deposition element and the temperature of said reaction vessel, and thereafter markedly changing the preliminary operating temperatures of said dissociation-deposition element and said reaction vessel to effect a change in the temperature differential between said dissociation-deposition element and said reaction vessel of the order of from 100 to 300 C., and continuing the progressive deposit of chromium until a substantial body of chromium has been formed, whereby a structurally weak separatory zone is formed between the initial thin layer of chromium and the substantial body of deposited chromium, and separating said subsequently deposited chromium at said separatory zone.

5. In a method for obtaining substantially pure chromium by contacting an atmosphere of heated chromium iodide vapor maintained within a heated reaction vessel with a dissociation-deposition surface maintained at a dissociation temperature for chromium iodide and thereby causing a body of substantially pure elemental chromium to be progressively deposited on such surface, the improvement that comprises allowing an initial thin layer of chromium to be deposited upon said dissociation-deposition surface under a preliminary set of operating conditions with respect to the temperature of said dissociation-deposition element and the temperature of said reaction vessel and thereafter abruptly changing the preliminary operating temperatures of said dissociationdeposition element and said reaction vessel to effect a 10 mium until a substantial body of chromium has been '1 formed, whereby a structurally weak separatory zone is formed between the initial thin layer of chromium and the substantial body of deposited chromium, and separating said substantial body of chromium at said sepa- 5 ratory zone.

References Cited in the fileof' this patent UNITED STATES PATENTS 1,567,079 Porzel Dec. 29, 1925 2,551,341 Scheer May 1, 1951 2,856,334 Topelian Oct. 14, 1958 OTHER REFERENCES Campbell et al.: Trans. of the Electrochemical Society, vol. 96, No. 5, 1949, pages 318433. 

1. IN A PROCESS FOR THE PRODUCTION OF PURE CHROMIUM BY DISSOCIATION AND DEPOSITION FROM CHROMOUS IODIDE IN CONTACT WITH A HEATED DISSOCIATION-DEPOSITION ELEMENT WITHIN A HEATED REACTION VESSEL, THE IMPROVEMENT THAT COMPRISES DEPOSITING AN INITIAL THIN LAYER OF CHROMIUM OR SAID DISSOCIATION-DEPOSITION ELEMENT UNDER A PRELIMINARY SET OF OPERATING CONDITIONS WITH RESPECT TO THE TEMPERATURE OF SAID DISSOCIATION-DEPOSITION ELEMENT AND THE TEMPERATURE OF SAID REACTION VESSEL, AND THEREAFTER ABRUPTLY CHANGING THE PRELIMINARY OPERATING TEMPERATURES TO EFFECT A CHANGE IN THE TEMPERATURE DIFFERENTIAL BETWEEN SAID DISSOCIATION-DEPOSITION ELEMENT AND SAID REACTION VESSEL OF THE ORDER OF FROM 100* TO 300*C. TO ALTER THE CRYSTAL GROWTH HABIT OF THE DEPOSITING CHROMIUM, WHEREBY A SEPARATORY WEAKENED ZONE IS FORMED BETWEEN THE INITIAL THIN LAYER OF CHROMIUM AND CHROMIUM SUBSEQUENTLY DEPOSITED ON SAID DISSOCIATION-DEPOSITION ELEMENT, AND SEPARATING SAID SUBSEQUENTLY DEPOSITED CHROMIUM AT SAID SEPARATORY WEAKENED ZONE. 